This invention relates to a heat transfer sheet, more particularly to a heat transfer sheet which is useful in the heat transfer system by use of a sublimable dye (heat migratable dye), excellent in peel-off prevention, etc. of the dye layer during heat transfer, and can also give excellent image density.
As the method which can give excellent mono-color or full-color (multicolor) image simply and at high speed in place of inking methods or printing methods in general of the prior art, there have been developed the ink jet system or the heat transfer system. Among them, as the method which can give a full-color image comparable with color photography having excellent continuous gradation, the so-called sublimation heat transfer system by use of a sublimatable dye is the most excellent. As the heat transfer sheet to be used in the above sublimation type heat transfer system, there has been generally used one having a dye layer containing a sublimable dye formed on one surface of a base film such as polyester film, and also having on the other hand a heat-resistant layer provided on the other surface of the base film for prevention of sticking of thermal head.
By superposing the dye layer surface of such heat transfer sheet on an image-receiving material having an image-receiving layer comprising a polyester resin, and heating imagewise by a thermal head from the back surface of the heat transfer sheet, the dye in the dye layer is migrated to the image-receiving material to form a desired image.
In the heat transfer system as described above, although there is an excellent advantage that it is possible to obtain the graduation of an image depending on the temperature of the thermal head, if the temperature of the thermal head is elevated in order to make higher the density, there will occur inconvenience that the binder forming the dye layer is softened thereby to be stuck onto the image-receiving material, whereby the heat transfer sheet is adhered to the image-receiving material, and further in an extreme case, there ensues the problem that the dye layer itself will be peeled off during peeling of them and transferred as such onto the image-receiving material surface.
Also, as the method for enhancing image density, there is the method in which the dye concentration in the dye layer is enhanced, but in this case, the binder content in the ratio of the dye and the binder in the dye layer becomes smaller, and as the result there ensues a similar problem as described above.
As the method for solving such problems, there has been proposed a method to provide an adhesive layer comprising a conventional adhesive resin such as polyurethane or polyester between the base film and the dye layer.
However, the method of providing such an adhesive is accompanied with such problems as mentioned below.
(1) Since the dye layer is formed by use of a composition containing a dye and a binder dissolved or dispersed in an organic solvent, the organic solvent in the composition for formation of dye layer dissolves the above adhesive layer formed on the base film during formation of the dye layer, whereby the function of the adhesive layer cannot be sufficiently fulfilled.
(2) If the thickness of the adhesive layer is made larger for solving the above problem (1), heat imparting efficiency from the thermal head to the dye layer will be remarkably lowered.
(3) Also, as the result of thickening of the adhesive layer, the dye in the dye layer will be also heat migrated during recording, with the result that no sufficient image density can be obtained.